The present invention is due to the collaboration of the Service National des Champs Intenses (Director Mr. Guy AUBERT) and it relates to a method of adjusting the homogeneity correctors of the magnetic field created by a magnet. It finds its application particularly in the medical field where magnets are used for carrying out non invasive investigations by applying nuclear magnetic resonance image forming methods. It may nevertheless find its application in other fields, particularly in that of scientific research where intense fields produced by magnets are used.
1. Field of the Invention
Nuclear magnetic resonance is a phenomenon of oscillation of the magnetic moment of the nuclei of the atoms or molecules of a body at a frequency which depends on the intensity of a magnetic field in which this body bathes. It follows from this statement that if the magnetic field varies, the frequencey of the resonance phenomenon varies also. Thus, for technological and technical reasons, in such experimentation, it is of the highest importance for the field produced by the magnet to be very homogeneous in its zone of interest. The required homogeneity is generally of the order of a few parts per million in the medical field, even of a few parts per billion in the scientific field. With this end in view, attempts have been made to construct magnets giving the most perfectly homogeneous field.
Unfortunately, whatever the care taken in the construction of the magnets, their construction is never as perfect as the theory which led to their calculation. Moreover, even if this defect can be removed, the magnet must materially be placed in a given position in order to be used. Now, none of the regions in space on earth, in an industrial or urban environment, is totally free of disturbing magnetic elements. The result is that, once installed on the site, the field produced in the useful zone of a magnet presents inhomogeneities which it is now necessary to try and correct.
2. Description of the Prior Art
The principle in correcting field inhomogeneities is that of superimposition: coils, magnetic pieces or any other means for producing magnetic fields are added so as to correct the imperfections of the main field and so as to produce an overall homogeneous field. The theory of the correction was originally developed by Messrs. SAUZADE M. and KAN S. K. in the review "Advances in Electronics and Electron Physics", vol. 34, 1973 Academic Press and their work is recounted in the collection of "Techniques de l'Ingenieur" chapter E 4351. The authors started from the principle that a magnetic field, at any point in space, may be expressed mathematically in the form of a so called analytic expression of the type B.sub.Z (x,y,z)=f(x,y,z). In this expression B.sub.Z represents the main component of the field at a coordinate point x,y,z and f is the analytic function used. The function f that these authors explained is written in the form of a polynomial breakdown into x,y,z of order or power increasing from 0 to infinity. Under certain restrictions, particularly the nearness of the regions concerned to the correction means which modify the field thereof, the high order terms of this expression may be neglected.
These authors were able to derive from this breakdown a general principle of independence of the corrections. This independence means that it is possible to construct correction means acting on the higher orders, but not on the lower, within a given order. For example, on pages 4-6 of the above mentioned collection, these authors describe a series of 12 coils for correcting successively field inhomogeneities up to the orders 4 of the analytic expression in question. Thus, for example after correcting the first order inhomogeneities of the main field with a first coil designed for this purpose, second order inhomogeneities of the main field may be corrected, to which are added meanwhile second order inhomogeneities due to the first coil, by means of a second correction coil. The purpose of this second coil is to correct the second order inhomogeneities but not to produce first order inhomogeneities. And so on, the correction is made successively for all the orders. For the adjustment, after each correction, before passing to the next correction, the field is remeasured. Then the correction to be made is estimated by comparing the field value defects at measuring positions with fields which may be obtained with the next coil. This procedure is laborious, it only gives good results after numerous attempts. Moreover, the adjustment is never achieved under the best conditions.
In their complexity, the theory and adjustment are simplified by a possibility of differenciation between so called axial corrections, those which correspond to the variations of the main field which are met with moving parallel to this field in the useful zone, and so called radial corrections which correspond to the inhomogeneities which are met with when moving perpendicularly to the orientation of this main field. It is clear from this study that the axial and radial corrections are independent of each other, at least if only the component of the main field oriented in the desired orientation of this field is taken into account. The above mentioned authors thus constructed, in one example, 12 coils adapted for forming a complete correction system. This system is universally known. It is used by many constructors.
More recently, Mrs. F. ROMEO and Mr. D. I. HOULT published in the review Magnetic Resonance in Medicine 1, 44-65 of 1984 an article entitled "Magnetic Field Profiling: Analysis and Correcting Coil Design". The work of these authors is complementary to that of the first ones. They show in particular that for the analytic expression of the field the polynomial breakdown based on carthesian coordinates is not the best. On the contrary, the sperical coordinates which lead to an analytic expression of the field in the form of a combination of spherical harmonics are more propitious to the understanding aand formation of typical structures for correction means to be constructed. The latter authors further teach a correction methodology. This methodology includes the following steps: first of all the main field to be corrected is measured. This result may be obtained using convetional means. Once measured, an a analytic expression therefor is sought in the preferred form. From this analytic expression, which is compared with an ideal analytic expression (representing an ideal field corresponding, for example to a constant B.sub.Z), is derived the analytic expression of a corrector field to be produced. Finally, by considerations similar to those which prevailed for the preceding authors, it is proposed to construct coils, or any other correction means, so as to reach the desired homogeneity of the main field. This erudite methodology has however a major drawback: it is not industrial. In fact, it assumes that the correction systems must be calculated, manufactured and installed specifically for each magnet. Technically and financially this methodology is inapplicable outside the laboratory. On the one hand because it corresponds to a new craft not yet represented in industry: that of calculating and constructing correction coils on request; and on the other hand because even in this case the cost and time spent in such constructions would be prohibitive.